Reproducible builds enable anyone to reproduce bit by bit identical binary packages from a given source, so that anyone can verify that a given binary derived from the source it was said to be derived.
There is more information about reproducible builds on the Debian wiki and on https://reproducible-builds.org.
These pages explain in more depth why this is useful, what common issues exist and which workarounds and solutions are known.

Reproducible Arch Linux is an effort to apply this to Arch Linux. Thus Arch Linux packages are build twice, with a few variations added and then the resulting packages from the two builds are compared using diffoscope.
Please note that this is still at an early stage. Also there are more variations expected to be seen in the wild.
Missing bits for testing Arch Linux: